The present invention relates to electronic control and sensing circuitry for oscillating conduit Coriolis-type mass flowmeters.
A mass flowmeter is an instrument which provides a direct indication of the quantity or mass, as opposed to volume or velocity, of material being transferred through a pipeline.
One class of mass measuring flowmeters is based on the well-known Coriolis effect. Coriolis forces are exhibited in the radial movement of mass on a rotating body. Imagine a planar surface rotating at constant angular velocity about an axis, perpendicularly intersecting the surface. A mass travelling radially outward on the surface at what appears to be a constant linear speed actually speeds up in the tangential direction because the larger the radial distance of a point from the center of rotation, the faster the point must travel. The increase in velocity, however, means that the mass has been indirectly accelerated. The acceleration of the mass generates a reaction force, called the Coriolis effect, in the plane of rotation perpendicular to the instantaneous radial movement of the mass. In vector terminology, the Coriolis force vector is the cross-product of the angular velocity vector (parallel to the rotational axis) and the velocity vector of the mass in the direction of its travel with respect to the axis of rotation (e.g., radial). Consider the mass as a person walking a straight line radially outward on a turntable rotating clockwise at a constant rate and the reaction force will be manifested as a listing to the left to compensate for acceleration.
The potential applicability of the Coriolis effect to mass flow measurement was recognized long ago. If a pipe is rotated about a pivot axis orthogonal to the pipe, material flowing through the pipe becomes a radially travelling mass which, therefore, experiences acceleration. The Coriolis reaction force experienced by the travelling fluid mass is transferred to the pipe itself as a deflection or offset of the pipe in the direction of the Coriolis force vector in the plane of rotation.
Coriolis-type mass flowmeters induce a Coriolis force in two significantly different ways: by continuously rotating or by oscillating back and forth. The principal functional difference is that the oscillating version, unlike the continuously rotating one, has periodically (i.e., usually sinusoidally) varying angular velocity producing, as a result, a continuously varying level of Coriolis force. A major difficulty in oscillatory systems is that the Coriolis effect is relatively small compared not only to the drive force but even to extraneous vibrations. On the other hand, an oscillatory system can employ the bending resiliency of the pipe itself as a hinge or pivot point for oscillation and thus obviate separate rotary or flexible joints and moreover offers the possibility of using the resonant frequency of vibration to reduce drive energy.
Several aspects of oscillating conduit Coriolis mass flowmeters require electronic instrumentation. First, inducing the oscillation in the conduit requires a drive control system sensitive to spurious vibration and capable usually of maintaining a constant amplitude as well as frequency of oscillation. Second, the movement of the conduit has to be detected and measured in such a way as to reveal the amount of the extraneous deflection or offset of the conduit due exclusively to Coriolis force. This application presents electronic instrumentation for implementing drive control and sensing functions in oscillating conduit Coriolis mass flowmeters.